Fuel rods with certain amount arranged in a predetermined spacing (such as 15×15, 17×17.etc.) and formed to a bunch are called as a nuclear reactor fuel assembly which mainly includes a nozzle, a top nozzle, a bottom nozzle, spacer grids, guide thimbles and fuel rods. A conventional nuclear reactor fuel assembly will be maintained in the nuclear reactor for 3˜5 years under a rigorous operation condition, since it is placed in a strong neutron field and must suffer high temperature, high pressure, and strong scouring of high-speed coolant flow, and suffer chemistry action of fission products and complex mechanical loads meanwhile.
As well known, the main intention of the multiple protections for the nuclear reactor is to prevent large quantity of radioactive substances from being leaked out from the nuclear reactor once explosion accidents of nuclear reactor happen. While the important factors for preventing explosion accidents are to control the speed and the temperature of the chain reaction in the nuclear reaction, thus the water flow control of the light water in the fuel assembly that is acted as moderator and coolant is critical, and the structure of the bottom nozzle of the fuel assembly is quite important to the water flux control of the light water. Specifically, the bottom nozzle is required to configure a skirt matching with spacer grids on which fuel rods are placed, and the skirt is provided with flow holes to allow the light water to flow, such flow holes must be sufficient and have enough strength to suffer the weight of the spacer grids and the scouring of the high-speed water flow. In addition, the flow holes must be sized to filter the foreign substance in the light water.
A skirt of a conventional bottom nozzle is provided with multiple flow holes and connecting holes, and predetermined spacing must be formed among the holes to make the skirt have enough strength, as a result, most entity area of the skirt may obstruct a part of water flowing through the bottom nozzle, thus the water speed after water flow passing through the bottom nozzle is reduced, as a result, large water pressure drop is generated, which may cause accidents of nuclear reactor. Note, the water flow direction is from down to up, namely from the lower end of the bottom nozzle to the upper end thereof.
Thus, there is a need to provide an improved bottom nozzle that has improved filter capacity and generates smaller water pressure drop.